Coupling between proteolytic processing and translocation of the precursor of the F1-ATPase beta-subunit during its import into mitochondria of intact cells

Effects of rhodamine 6G on the mitochondrial ultrastructure of mouse spermatocytes

Effects of rhodamine 6G on spermatocyte mitochondria were assessed in BALB/c mice in vivo. Four groups of mice were given rhodamine 6G doses of 0.0, 0.3 and 0.5 and 0.8 mg.kg-1.d-1 i.p. for 5 days. On the 5th day after the last day of dye administration, the mice were killed and testicular tissue was prepared for electron microscopy. Exposure to rhodamine 6G resulted in dose-dependent effects, primarily intracristal expansion of mitochondria.

A signal sequence domain essential for processing, but not import, of mitochondrial pre-ornithine carbamyl transferase

Studies using deletion mutagenesis indicate that a processing recognition site lies proximal to the normal cleavage position between gln32 and ser33 of pre-ornithine carbamyl transferase (pOCT). pOCT cDNA was manipulated to delete codons specifying amino acids 22-30 of the signal sequence. The mutant precursor, designated pOCT delta 22-30, was imported to the matrix compartment by purified mitochondria, but remained largely unprocessed; the low level of processing that was observed did not involve the normal cleavage site. Several manipulations performed downstream of the normal pOCT processing site (deletion, substitution, and hybrid protein constructions) affected neither import nor correct processing. Our data suggest that domains specifying import and processing site recognition may be functionally segregated within the signal peptide; that processing is not requisite for import of pOCT; and that a proximal region, not involving the normal signal peptide cleavage site, is required for processing site recognition.

Protein translocation across and integration into membranes

This review concentrates mainly on the translocation of proteins across the endoplasmic reticulum membrane and cytoplasmic membrane in bacteria. It will start with a short historical review and will pinpoint the crucial questions in the field. Special emphasis will be given to the present knowledge on the molecular details of the first steps, i.e., on the function of the signal recognition particle and its receptor. The knowledge on the signal peptidase and the ribosome receptor(s) will also be summarized. The various models for the translocation of proteins across and the integration of proteins into membranes will be critically discussed. In particular, the function of signal, stop-transfer, and insertion sequences will be dealt with and molecular differences discussed. The cotranslational mode of membrane transfer will be compared with the post-translational transport found for mitochondria and chloroplasts. This review will conclude with open questions and an outlook.